Currently available lightning arrestors for installation in residential circuits typically utilize a voltage-dependent resistor, such as a silicon carbide varistor, in series with a spark gap. The presence of the spark gap is prescribed since currently available silicon carbide varistors suitable for lightning arrestor application, being connected from line to neutral in a service entry panelboard, would otherwise draw excessive current at normal line voltage and thus heat up to the point of ultimately destroying itself. Failure of this silicon carbide varistor could then constitute a short circuit with damaging consequences. Since the spark gap has a typical breakdown voltage of 2,000 volts, the arrestor affords no overvoltage protection for voltage surges below this level. While such low level voltage surges do not pose significant harm to wiring and most connected loads, they are potentially damaging to electronic circuitry, particularly solid state electronic circuitry currently utilized in televisions, radios and audio equipment.
My U.S. Pat. No. 4,168,514 discloses a combination circuit breaker-lightning arrestor which utilizes a metal oxide varistor (MOV) directly electrically connected, i.e., no spark gap, from ground to a line connector at a point downstream from the breaker contacts and thermal trip element (bimetal). A suitable MOV for 120 volt circuit application should have a nominal rating of 220 volts at one milliampere and be capable of dissipating at least 20 joules and preferably 40 joules or more of electrical energy. Under these circumstances the MOV is capable of affording complete overvoltage protection against even relatively low level voltage surges. As a failsafe measure, the MOV is thermally coupled with the breaker trip element, such that, should the MOV go into a failure mode evidenced by the conduction of excessive leakage current under normal line voltage conditions, the consequent heat developed by the MOV ulitimately effects thermal tripping of the circuit breaker. The breaker contacts thus open to interrupt the flow of excessive MOV leakage current before the MOV goes to complete failure. At this point, the combination circuit breaker-lightning arrestor should be replaced, since, although a failing MOV can still afford voltage surge suppression, the closure of the circuit breaker would obviously result in reheating of the MOV and again thermal tripping of the circuit breaker. Repeated reclosures of the circuit breaker accelerates the MOV to complete failure with potentially hazardous consequences.
It is accordingly an object of the present invention to provide an improved lightning arrestor.
Another object is to provide a lightning arrestor of the above character which is normally capable of suppressing low level voltage surges potentially damaging to solid state electronic circuitry.
A further object is to provide a lightning arrestor of the above character which utilizes the voltage surge suppression potential of a failing voltage-dependent resistor to maintain overvoltage protection, albeit limited to higher level voltage surges.
Yet another object is to provide a lightning arrestor of the above character which automatically in response to a failing voltage-dependent resistor converts from a low level voltage surge suppressor to a higher level voltage surge suppressor.
A still further object is to provide a lightning arrestor of the above character which is simple in construction, inexpensive to manufacture and reliable in operation.
Other objects of the invention will in part be obvious and in part appear hereinafter.